The present application relates generally to current sensing devices for electrical systems, and more particularly to fault indicators for detecting ground fault currents in alternating current power distribution systems.
Various types of fault indicators have been constructed for detecting fault currents in individual conductors of alternating current power distribution systems, including clamp-on type fault indicators, which clamp directly over cables in the system, and test-point type fault indicators, which are mounted over test points in the cables or associated connectors of the system. Such fault indicators may be either of the manually reset type, wherein it is necessary that the indicator be physically reset following each fault, or of the automatically reset type, wherein a fault indicator is reset upon restoration of line current. Examples of such fault indicators are found in products manufactured by E. O. Schweitzer Manufacturing Company of Mundelein, Illinois, and described in U.S. Pat. Nos. 3,676,740, 3,906,477 and 4,063,171, of the present inventor.
It is sometimes necessary that multi-phase mains in power distribution systems be monitored for ground faults, wherein current flows from a load directly to ground and does not return through the individual neutral or phase conductors of the system. Fault indicators such as those described above, which respond to fault currents in a single monitored conductor, are not suitable for the purpose of detecting ground fault currents, which may be lower than the normal current in any one phase conductor.
One approach at detecting ground faults in multi-phase mains has been to lead the individual phase conductors, and the neutral conductor, where present, through a toroidal summation current transformer on which a secondary sense winding is provided. In the absence of a ground fault the vector sum of all currents in the phase conductors extending through the summation current transformer is zero, and no current is induced in the sense winding. In the event of a ground fault, the sum of the currents flowing through the transformer is no longer equal to zero, and a current is induced in the sense winding. This current is amplified and processed in amplification and processing circuitry to produce an output signal indicative of the occurrence of a ground fault.
Unfortunately, the sense winding and associated amplification and processing circuitry add to the cost and complexity of such toroidal core ground fault indicators, making the indicators unsuitable for many applications, particularly those at remote locations where environmental and cost considerations dictate a small easily interpreted self-contained unit operable from available power sources of relatively high voltage. Furthermore, the toroidal core is difficult to install on existing multi-phase mains, since the individual phase conductors must be disconnected to enable the mains cable to be routed through the closed toroidal core.
The present invention is directed to a new and improved ground fault indicator which does not require a toroidal core having a sense winding, and which is operable from existing voltage sources in a power distribution system.
Accordingly, it is a general object of the present invention to provide a new and improved ground fault indicator.
It is a more specific object of the present invention to provide a ground fault indicator which is compact and economical to construct and which does not require a toroidal core having a sense winding.
It is a still more specific object of the present invention to provide a ground fault indicator which can be readily installed and removed from a multi-phase mains cable of a power distribution system.